Electron spin echo envelope modulation study of oxygenated iron-cobalt hybrid hemoglobins reveals molecular features analogous to those of the oxy ferrous protein.

ABSTRACT

Two oxygenated iron-cobalt hybrid hemoglobins (Hbs), (alpha Co-O2 beta Fe-O2)2 and (alpha Fe-O2 beta Co-O2)2, were studied by electron spin echo envelope modulation (ESEEM) spectroscopy in order to measure (i) electron-nuclear hyperfine and nuclear quadrupole coupling to the N epsilon of the proximal histidyl imidazole and (ii) nuclear hyperfine coupling to exchangeable 2H in the oxyCo subunits. 14N couplings were found to be smaller in the oxyCo alpha subunits than in the oxyCo beta subunits, suggesting a more ionic and shorter Co-O2 bond in the alpha subunits [Lee et al. (1994) Biochemistry 33, 7609], which correlates with the higher O2 affinity found for (alpha Co beta Fe-O2)2 Hb than for (alpha Fe-O2 beta Co)2 Hb [Imai et al. (1980) J. Mol. Biol. 138, 635]. A smaller nuclear quadrupole coupling constant found for the proximal histidyl N epsilon in the oxyCo alpha subunits also suggests an increase in the overlap between the N epsilon sp2 hybrid and the Co dz2 orbital, i.e., a shorter Co-N epsilon bond, than in the oxyCo beta subunits. On the other hand, the relative orientation of the g and 14N epsilon nuclear quadrupole tensors, obtained by spectral simulation, suggests that the Co-O-O bond angle is similar in the two types of oxyCo subunits. An X-ray crystallographic study of oxyFe Hb A [Shaanan, B. (1982) Nature 296, 683] has also reported similar Fe-O-O bond angles in both alpha and beta subunits, but with shorter Fe-N epsilon and Fe-O2 bonds in the alpha subunits.(ABSTRACT TRUNCATED AT 250 WORDS)